Preserve context values

Instead of passing a fresh `context.Background()` wrap context passed by
the user with a context that ignores cancellation.

This allows to drop the original cancellation while preserving context
values.

If built with Go 1.21+ the stdlib `context.WithoutCancel` is used.
Otherwise it fallbacks to an in-tree copy of the withoutCancel.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

singleflight.go

@@ -25,10 +25,12 @@ type Group[K comparable, V any] struct {
 // comes in, the duplicate caller waits for the original to complete and
 // receives the same results.
 //
-// The context passed to the fn function is a new context which is canceled when
-// contexts from all callers are canceled, so that no caller is expecting the
-// result. If there are multiple callers, context passed to one caller does not
-// effect the execution and returned values of others.
+// The context passed to the fn function is a context that preserves all values
+// from the passed context but is cancelled by the singleflight only when all
+// awaiting caller's contexts are cancelled (no caller is awaiting the result).
+// If there are multiple callers, context passed to one caller does not affect
+// the execution and returned values of others except if the function result is
+// dependent on the context values.
 //
 // The return value shared indicates whether v was given to multiple callers.
 func (g *Group[K, V]) Do(ctx context.Context, key K, fn func(ctx context.Context) (V, error)) (v V, shared bool, err error) {
@@ -45,7 +47,9 @@ func (g *Group[K, V]) Do(ctx context.Context, key K, fn func(ctx context.Context
 		return g.wait(ctx, key, c)
 	}
 
-	callCtx, cancel := context.WithCancel(context.Background())
+	// Replace cancellation from the user context with a cancellation
+	// controlled by the singleflight and preserve context values.
+	callCtx, cancel := context.WithCancel(withoutCancel(ctx))
 
 	c := &call[V]{
 		done:    make(chan struct{}),

singleflight_test.go

@@ -393,6 +393,26 @@ func TestDo_multipleCallsCanceled(t *testing.T) {
 	}
 }
 
+func TestDo_preserveContextValues(t *testing.T) {
+	var g singleflight.Group[string, any]
+
+	type KeyType string
+	const key KeyType = "foo"
+
+	callerCtx := context.WithValue(context.Background(), key, "bar")
+
+	val, _, err := g.Do(callerCtx, "key", func(ctx context.Context) (any, error) {
+		return ctx.Value(key), nil
+	})
+
+	if err != nil {
+		t.Fatal(err)
+	}
+	if val != "bar" {
+		t.Error("the context should not lose the values")
+	}
+}
+
 func waitStacks(t *testing.T, loc string, count int, timeout time.Duration) {
 	t.Helper()
 

withoutcancel.go

@@ -0,0 +1,87 @@
+//go:build !go1.21
+
+// Copyright (c) 2009 The Go Authors. All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//    * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//    * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//    * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+//
+// Source: https://cs.opensource.google/go/go/+/refs/tags/go1.21.1:src/context/context.go
+// The only modifications to the original source were:
+// - renaming WithoutCancel to withoutCancel
+// - replacing the usage of internal reflectlite with reflect
+// - replacing the usage of private value function with Value method call
+package singleflight
+
+import (
+	"context"
+	"reflect"
+	"time"
+)
+
+// withoutCancel returns a copy of parent that is not canceled when parent is canceled.
+// The returned context returns no Deadline or Err, and its Done channel is nil.
+// Calling [Cause] on the returned context returns nil.
+func withoutCancel(parent context.Context) context.Context {
+	if parent == nil {
+		panic("cannot create context from nil parent")
+	}
+	return withoutCancelCtx{parent}
+}
+
+type withoutCancelCtx struct {
+	c context.Context
+}
+
+func (withoutCancelCtx) Deadline() (deadline time.Time, ok bool) {
+	return
+}
+
+func (withoutCancelCtx) Done() <-chan struct{} {
+	return nil
+}
+
+func (withoutCancelCtx) Err() error {
+	return nil
+}
+
+func (c withoutCancelCtx) Value(key any) any {
+	return c.c.Value(key)
+}
+
+func (c withoutCancelCtx) String() string {
+	return contextName(c.c) + ".WithoutCancel"
+}
+
+type stringer interface {
+	String() string
+}
+
+func contextName(c context.Context) string {
+	if s, ok := c.(stringer); ok {
+		return s.String()
+	}
+	return reflect.TypeOf(c).String()
+}

withoutcancel_go121.go

@@ -0,0 +1,12 @@
+//go:build go1.21
+
+package singleflight
+
+import "context"
+
+// withoutCancel returns a copy of parent that is not canceled when parent is canceled.
+// The returned context returns no Deadline or Err, and its Done channel is nil.
+// Calling [Cause] on the returned context returns nil.
+func withoutCancel(ctx context.Context) context.Context {
+	return context.WithoutCancel(ctx)
+}